Not applicable.
Not applicable.
The field of the invention is motor controllers and more specifically filtering systems for removing cyclic load disturbances from motor control systems.
An exemplary motor control system includes a controller and a motor drive. The drive is linked to a motor and provides currents to motor windings thereby causing a motor rotor to rotate within a stator construct. To this end, a controller typically receives one or more command signals that indicate intended motor operating characteristics. For instance, one exemplary intended characteristic may be rotor velocity which is received by the controller in the form of a command velocity signal. The controller is programmed to provide control signals to the drive to cause the motor to operate in accordance with the command signals. The drive receives the control signals and attempts to drive the motor in accordance therewith.
Unfortunately, many motor drive applications are characterized by mechanical and electrical disturbances that hamper the control process and cause the motor to operate in other than the commanded fashion. Some of the disturbances are non-cyclic while others are cyclic. For instance, an increased load is a change that occurs over an operating period and therefore is not cyclic. In contrast, mechanical resonances (e.g., a dual inertia motor-load system with a spring-like coupling that exists between the inertias) and electrical disturbances (e.g., system force induced by the mutual torques that exist in the harmonics associated with back electromotive forces (EMFs)) are typically cyclic and hence occur at system specific and recurring frequencies.
To minimize the affects of system disturbances, many control systems include one or more sensors and corresponding feedback loops. The sensors are configured and positioned so as to measure motor operating parameters and provide feedback signals to the controller. For instance, one feedback signal may include a feedback velocity signal. In the case of a velocity feedback system, the controller compares the command velocity signal to the feedback velocity signal and generates an error command signal for controlling the drive. The controller may simply subtract the feedback signal from the command velocity signal and use the difference as a velocity error signal for controlling the drive. Typically, to expedite control, the velocity error signal is manipulated (e.g., via a PI controller) prior to being fed to the drive.
Conventional feedback systems achieve their end results (i.e., reduce the affects of system disturbances) in a less than optimal fashion by overshooting and undershooting command signals and thereby cause system errors. For instance, where motor velocity is below a commanded velocity, conventional feedback control systems simply increase the velocity error signal until the feedback velocity is above the commanded velocity. Thereafter the feedback signal indicates that the motor velocity is higher than the commanded velocity and the controller reduces the velocity error signal until the motor velocity, as represented by the feedback signal, is again below the commanded velocity. This overshooting and undershooting process is repeated without end and never reaching constant steady state.
With respect to non-cyclic disturbances (e.g., a slowly changing load), conventional feedback controllers work relatively well as, after several cycles, a steady state condition should result. Unfortunately, in the case of cyclic disturbances, the target compensated signal must inversely mirror the changing disturbance and therefore the overshoot and undershoot problem persists.
For instance, assume a command velocity of 2 Hz and a cyclic disturbance including an 8 Hz component and a 16 Hz component. In this case, the combined feedback signal includes 2, 8 and 16 Hz components and therefore the velocity error signal is dynamic and includes 8 and 16 Hz components. Thus, despite the recurring cyclic nature of the disturbances, the conventional feedback system would be unable to eliminate the disturbances without generating persistent overshoot and undershoot related noise.
It has been recognized that, in the case of cyclic-type disturbances, the frequencies of the most prominent instantaneous cyclic disturbances can be determined and used to, in effect, anticipate the cyclic characteristics of subsequent disturbances, the anticipated characteristics thereafter being used to modify the velocity error signal that represents the difference between the commanded signal and the feedback signal. To this end, to anticipate subsequent characteristics of a cyclic disturbance, three characteristics of the disturbance have to be identified. First, the frequencies corresponding to the separate components of the disturbance have to be identified. Second, the phases of the separate components of the disturbance have to be identified. Third, the amplitudes of the disturbance components have to be identified. After frequencies, phases and amplitudes of the disturbance components are identified, a compensation signal mirroring the disturbance can be generated and subtracted from the manipulated velocity error signal to generate a corrected error signal that essentially anticipates the disturbance and compensates therefore.
To this end, an exemplary embodiment of the invention includes an apparatus for reducing cyclic noise in a motor drive system where the drive system subtracts a velocity feedback signal from a velocity command to generate a velocity error signal and uses the velocity error signal to control other drive system components, the apparatus comprising a frequency identifier that receives a velocity feedback signal and identifies unintended frequencies of unintended components of the feedback signal where each unintended component is characterized by a component specific phase and amplitude, a sine generator that generates a combined signal characterized by a separate sinusoidal component corresponding to each of the undesirable frequencies, a tuning signal identifier linked to at least one system component output and providing a tuning signal corresponding to the at least one component output, an LMS module that mathematically combines the combined signal and the tuning signal to generate a compensation signal characterized by a separate component corresponding to each identified unintended frequency where each separate component of the compensation signal is essentially in phase with and has an amplitude similar to a corresponding unintended component; and a regulator module that mathematically combines the velocity error signal and the compensation signal to generate a corrected signal used to control other drive components.
In at least one embodiment the sine generator includes a plurality of separate sine modules and a second summer, the transform module providing a separate one of the unintended frequencies to a different one of the sine modules, each sine module receiving an unintended frequency generating a sine wave signal at the unintended frequency, the sine wave signals provided to the second summer and the second summer mathematically combining the sine wave signals to generate the combined signal. The second summer may mathematically combine by adding the sine wave signals.
In at least one embodiment the frequency identifier includes a Fast Fourier Transformer (FFT) module that generates frequency signals corresponding to each frequency present in the feedback signal. The frequency identifier may further include a frequency discriminator that receives the frequencies generated by the FFT module and identifies a sub-set of the received frequencies as unintended frequencies.
The discriminator may also receive the command velocity signal and, when the command velocity signal is a square wave having a fundamental frequency, identifies odd multiple harmonics of the command velocity signal and identifies frequencies other than the fundamental frequency and odd multiples of the fundamental frequency that have magnitudes greater than a threshold magnitude as the unintended frequencies.
The LMS module may mathematically combine by altering the phase and amplitude of the combined signal as a function of the tuning signal. To this end, in at least one embodiment where the combined signal is an original combined signal, the LMS module includes N delays, N+1 weighting modules, N+1 integrators, N+1 multipliers and an LMS summer and wherein the delays are serially arranged to receive the combined signal and delay the combined signal to generate N separate and uniquely delayed combined signals, each of the original signal and the delayed signals provided to a separate one of the weighting modules and also to a separate one of the multipliers, each multiplier further receiving the tuning signal and mathematically combining the tuning signal and the corresponding delayed signal and providing an output signal to a separate one of the integrators, each integrator integrating the received output signal and providing a weight signal to a corresponding one of the weighting modules, each weighting module manipulating a corresponding one of the original or delayed signals to generate a weighted signal, the LMS summer receiving and mathematically combining each of the weighted signals to generate the compensation signal.
Each multiplier may mathematically combine by multiplying a corresponding one of the original and delayed signals and the tuning signal.
The LMS module may further include a rate gain module that manipulates the tuning signal by a factor k and provides the manipulated error signal as the tuning signal to each of the multipliers. The regulator module in some embodiments includes a PI regulator and a summer and wherein the PI regulator receives and manipulates the velocity error signal to generate a modified error signal and the summer subtracts the compensation signal from the modified error signal to generate the corrected signal. Here, the apparatus may further include a derivative module that receives the velocity feedback signal and converts the velocity feedback signal into an acceleration feedback signal and, wherein, the acceleration feedback signal is the tuning signal. In the alternative, either the corrected signal or the velocity error signal may be the tuning signal.
Moreover, in some embodiments the regulator module further includes a summer that subtracts the compensation signal from the velocity error signal to generate an intermediate corrected signal and a PI regulator that receives and modified the intermediate corrected signal to generate the corrected signal and, wherein, the intermediate corrected signal is the tuning signal.
The invention further includes a method for reducing harmonic noise in a motor drive where the drive subtracts a velocity feedback signal from a velocity command to generate a velocity error signal and uses the velocity error signal to control other drive components. The method comprised the steps of obtaining a system velocity feedback signal, identifying unintended frequencies of unintended components of the feedback signal where each unintended component is characterized by a component specific phase and amplitude, generating a combined signal characterized by a separate sinusoidal component corresponding to each of the undesirable frequencies, identifying a tuning signal corresponding to at least one system component output, mathematically combining the combined signal and the tuning signal to generate a compensation signal characterized by a separate component corresponding to each identified unintended frequency where each separate component of the compensation signal is essentially in phase with and has an amplitude similar to a corresponding unintended component and mathematically combining the velocity error signal and the compensation signal to generate a corrected signal used to control other drive components.
The step of generating the combined signal, in some embodiments, includes the steps of generating separate sine wave signals at each of the unintended frequencies and mathematically combining the sine wave signals to generate the combined signal. Here, the step of mathematically combining the sine wave signals may include the step of adding the sine wave signals.
The step of identifying may include the step of performing a Fast Fourier Transformer (FFT) on the feedback signal. Here, the step of identifying further may include, when the command velocity signal is a square wave having a fundamental frequency, identifying odd multiple harmonics of the command velocity signal and identifying frequencies other than the fundamental frequency of the command velocity signal and odd multiples of the fundamental frequency that have magnitudes greater than a threshold magnitude as the unintended frequencies.
In some embodiments the step of mathematically combining the combined signal and the tuning signal includes altering the phase and amplitude of the combined signal as a function of the tuning signal. Here the combined signal may be an original combined signal and the step of altering the phase and amplitude may include the steps of delaying the combined signal to generate N separate and uniquely delayed combined signals, for each of the original combined signal and delayed combined signals: (i) mathematically combining each of the original and delayed combined signals with the tuning signal to generate an output signal, (ii) integrating the output signal to generate a weight signal and (iii) mathematically combining the weight signal and the corresponding combined signal to generate a weighted signal and mathematically combining the weighted signals to generate the compensation signal.
In some embodiments the step of mathematically combining the velocity error signal and the compensation signal includes providing the error signal to a PI regulator that manipulates the velocity error signal to generate a modified error signal and subtracting the compensation signal from the modified error signal to generate the corrected signal.
Moreover, the apparatus for performing the methods described above may include a processor running a pulse sequencing program.
In addition to the apparatus and methods described above, it has been recognized that particularly advantageous disturbance reduction and elimination can be accomplished throughout the entire possible frequency range of disturbances by combining the inventive LMS filter configuration with other filters and disturbance eliminators. To this end, it has been recognized that the disturbance spectrum can generally be divided into three separate ranges including a low frequency range (e.g., 1-3 Hz.), a middle frequency range (e.g., 3-30 Hz.) and a high frequency range (e.g., 30 Hz-1 kHz). Generally, given typical system operation all frequency components within the high frequency range will comprise unintended system disturbances and therefore a notch filter tuned to eliminate high frequency components appreciably reduces the work required of the LMS filter.
At low frequencies, while the LMS filter may operate to eliminate unintended frequency components, it has been found that because of the low frequency of these components, the LMS filter becomes burdened by these components and may not operate in an ideal fashion. This is particularly true where the low frequencies of disturbances cause the disturbances to be similar to DC values. Thus, in some cases it is advantageous to include a low frequency range disturbance eliminator of some type to alleviate the LMS filter from having to remove low frequency disturbances.
Thus, at least some embodiments of the invention are also for substantially reducing the entire range of possible system noise and further include a notch filter that receives the velocity error signal and filters out resonant high frequency components of the velocity error signal thereby generating a modified velocity error signal and wherein the summer mathematically combines the modified velocity error signal and the compensation signal to generate the corrected signal.
Some embodiments further include a load estimator and a load estimator summer, the estimator also receiving the system feedback signal and a modified control signal and combining the received signals to generate a low frequency compensation signal, the estimator summer mathematically combining the corrected signal and the low frequency compensation signal to generate the modified control signal, the modified control signal also provided to the other drive components for control purposes. The frequency identifier may limit unintended components to frequencies within a middle frequency range (e.g., 3 through 30 Hz).
While advantageous to include each of a notch filter, an LMS filter and a load estimator in a system configuration, some embodiments may include an LMS filter and a load estimator without a notch filter or may include a notch filter and an LMS filter without an estimator. In addition, while the embodiment described herein teaches that the LMS filter feeds the load estimator, the order of the LMS filter and the estimator may be reversed.
These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.